Of the roughly 25,000 miles of gas distribution piping installed by the United States gas distribution industry in 1988, approximately 23,000 was plastic pipe and more specifically polyethylene pipe.
This increasing popularity of polyethylene pipe is due to its reduced installation cost, ease of joining, light weight and resistance to corrosion. While most existing methods for joining polyethylene pipe have a very good reputation for reliability, there is still a degree of craft sensitivity which requires very skilled workers and extensive training. Even with this training, a certain percentage of bad joints are made due to environmental conditions, equipment problems, installer errors or just bad luck.
While this percentage is fortunately very small, every leak, especially a major rupture is potentially very dangerous and most leaks cost a great deal to repair. Any reduction in craft sensitivity and increase in reliability would reduce the operating cost of the gas distribution system and increase its overall safety.
The recent development of electrofusion fittings for joining polyethylene pipe is a step in the right direction toward reducing the need for extensive fusion training and for reducing the cost of repairs and tie-ins. These electrofusion systems, due to the nature of their relatively complex design, are still very expensive and have a number of performance issues that limit their ultimate general acceptance. In the meantime, Butt fusion, socket fusion and mechanical fittings are still widely used.
When done correctly with proper times, temperatures and pressures, butt fusion using CAL ROD resistance heaters or the like works very well. Over the past 20 years, millions of butt fusion joints have been made with a low percentage of problems. However, every gas utility company has their horror stories of catastrophes where a series of bad butt welds created major problems and expense.
As to socket fusion, many of the comments on craft sensitivity and required training apply to this form of fusion joining as well.
The heater irons employed in socket fusion take a long time to heat up and if proper allowances aren't made for ambient temperature or if the installer is too slow between heating and pressing together, a bad joint is still likely. If the pipe is not fully inserted into the socket, the fitting and section of pipe must be cut out often wasting hours of work.
Mechanical fittings are used typically from 1/2" to 11/4" pipes. They are usually much more expensive than socket fusion fittings, but can be quickly installed with a hand tool, thus saving labor expense. A number of different systems can be found on the market and their reputations for reliability vary from gas company to gas company.
Returning to electrofusion, it has been extremely popular in England and France, but has been slow to take off in the U.S. gas industry. It has made the most significant inroads for repairs and tie-ins and should begin to enjoy growth in branch and saddle situations and line extensions.
Electrofusion systems require much less training to install than the other systems discussed which is a major advantage as is their ability to be installed in cramped conditions where other fusion methods are very difficult. A number of shortcomings however, have prevented the rapid acceptance of this process such as it is still very expensive, and it is sensitive to pipe orientation, especially gaps between the ends of the pipes. These gaps remove material from under the sections of the heating coil adjacent to these gaps, causing the heating wires to overheat the plastic. The plastic in these regions is overheated and may flow causing the heater wires to contact one another and thus short out or crowd together and produce still additional hot spots. In a worst case this can cause a fire or explosion. For these reasons the more sophisticated power supplies for these systems have elaborate methods of shutting off the power to the coil if a problem is detected. If this happens the fitting and any other pipe or piping components it is attached to must be cut out and scrapped. This procedure is extremely expensive and time consuming. A number of different approaches have been proposed to overcome these problems, such as temperature control and the use of clamps on each pipe with a rigid bar extending between the clamps. There is still concern that eventually a leak might develop out along the continuous wires of the heater coil, especially if the wires have overheated.
A problem that is inherent in each of the prior art systems discussed above is that a continuous fusion of the plastics does not occur over unbroken extended regions of contact of the sleeve and pipes. In consequence there are numerous inclusions that can and often do cause nucleation of cracking leading to expensive repairs particularly where the pipes are buried underground.